In conventionally producing twisted strands of insulated conductors or cables either the strand supply or strand takeup has been retated about the axis of the strands to impart a unidirectional twist. More recently, methods and apparatuses have been devised for forming twisted strand units without the need for revolving the strand supply or takeup by the procedure of reversing the direction of twistlay from section to section. This has become known as S-Z type twisted with S referring to left-hand twists and Z referring to right-hand twists. It is usually performed with mutually spaced twister heads referred to as an accumulator.
The previous methods of forming S-Z twisted strand units such as strand pairs and quads may be placed into select categories. In one category the speed of linear advance of the strands and the speed of revolution of the mutually spaced twister heads about the strands are both maintained constant while the spacing occupied by the strands between the twisting heads is varied. This may be done by reciprocal movement of a twister head located between the two other twister heads which provides a twist accumulator of variable storage capacity that has become known as a breathing accumulator. Examples of S-Z twisting methods employing such variable storage accumulators are shown in U.S. Pat. Nos. 3,373,550 and 3,782,092.
Another method of forming S-Z twisted strand units is performed with a device that is known as a variable capacity, in-line accumulator, an example which is shown in U.S. Pat. No. 3,052,079. With this type of accumulator the twisting heads are moved in unison up and down an advancing line of strands while the speed of advance of the strands and the speed of revolution of the twister heads thereabout are both maintained constant.
A third category of methods used in forming S-Z twisted strand units employs a fixed storage type accumulator which effects S-Z twisting by varying the advance speed of the strands through the accumulator while maintaining the speed of revolution of the accumulator twisting heads constant about the strands. An example of this approach is shown in U.S. Pat. No. 3,507,108. A fourth category of methods for forming S-Z twists also utilizes an accumulator of fixed storage content type but which varies the relative speed of rotation of the twister heads themselves while maintaining the linear speed of advance of the strands constant. This category is represented by U.S. Pat. Nos. 3,823,536 and 4,006,582.
The just described methods of forming S-Z twisted strand units have been characterized by various individual attributes and limitations. For example, though use of the variable capacity, in-line accumulator is made without altering strand advance speed, it has not proven to be practical in operation due to the need for moving the entire accumulator itself up and down the line of strand advance in performing twister operations. The breathing accumulator has met a larger degree of success attributable in part to the fact that it is a relatively compact system that operates at low traverse speeds. The breathing accumulator however is relatively complex and limits strand advance speed which can only be alleviated by operating the breathing accumulator itself at such speed that it exerts an unacceptable high degree of tension on the strands. With those storage accumulators of the type that employ a variable line speed it is necessary either to accept such line speed variations, which usually interfers adversely with other manufacturing operations being formed on the strands in tandem with the twisting operation, or to employ external accumulators between such other in tandem operations and the twisting operation in order to provide a fixed advance speed through the other manufacturing stations.
Still other problems are encountered with fixed storage accumulators of the type capable of maintaining a constant line speed and which effect S-Z strand twisting by varying the relative speed of revolution of the accumulator twisting heads. Foremost among these is the fact that by varying the relative speeds of the twisting heads twists initially formed in the strands are later untwisted by the downstream head to some extent. This tends to work harden metallic strands rendering them less flexible and also leads to the development of twist nonuniformities due to the inherent difficulty in controlling the untwisting of previously twisted strands.
Recently, as shown in U.S. Pat. No. 3,941,166, another approach has been taken in S-Z twisting which attains several advantages of the just described prior art methods to the exclusion of several of their disadvantages. Here, a fixed storage accumulator is employed with spaced twister heads mounted for synchronous movement about the strands which are advanced through the accumulator at a constant line speed. The twister heads are driven first in one direction about the strands and then abruptly driven in the opposite direction each time a twisted strand segment passes through the accumulator. In this manner the strands are twisted half of their final twist by the first head and the remaining half by the second head.
Though the lastly described approach offers several advantageous features it passes twist reversal points through the entire accumulator. This action renders the strands susceptible to becoming untwisted at twist reversal points in the process which is particularly true here since the strands are passed through the accumulator in only a half twisted condition. It thus would be desirable to devise yet another method of forming S-Z twists that could utilize a fixed storage type accumulator operated at a constant strand line speed without performing an untwisting operation upon the strands or passing twist reversal points through the accumulator.